Typically, radio frequency identification (RFID) tags are secured on or in movable items. In certain instances, the tag may be placed on a container for a multiplicity of the same items, rather than on the items themselves. The identity of and perhaps other information relating to the tagged item is stored in its tag, and is transmitted by the RFID tag to a remote RFID interrogator, or reader, in response to a scan, query or command from a reader within the response range of the tag, i.e., a range suitable for RF communication between reader and tag. In its simplest form, the conventional RFID tag consists of a transponder and an antenna. Sometimes, the RFID tag itself is referred to as a transponder.
RFID tags may be either passive or active. A passive RFID tag lacks an internal self-sufficient power supply, e.g., a battery, and relies instead on the incoming RF beam (in this example, a query by the reader) to produce sufficient power in the tag's internal circuitry to enable the tag to transmit a response. In essence, the query induces a small electrical current in the tag's internal antenna, which serves as the power source that enables a reflected or backscattered response in a read-only mode. Accordingly, a passive RFID tag is generally limited in the amount of data that can be furnished in its response, e.g., an ID number and perhaps a small amount of additional data. But the absence of a battery leads to certain advantages, primarily that a passive tag can be fabricated at much less cost and with considerably smaller size than an active tag. Among other current uses, passive RFID tags may eventually replace the ubiquitous universal product code (UPC), the bar code strip found on myriad products in the stream of commerce—the imprinted strip requiring a line of sight optical scan to produce the UPC readout and the resulting computerized display or printout of price (at a point of sale of the bar-coded product) and other information regarding the product.
The on-board battery of an active RFID tag can give the tag a greater response range, along with greater accuracy, reliability and data storage capacity, but the active tag has the aforementioned disadvantages of cost and size relative to the passive tag. The battery itself is quite small by present day standards, but not enough to overcome the size disadvantage.
The principles of the present invention are equally applicable to passive and active RFID tags.
A typical conventional RFID tag reader employs a transceiver, a control unit and an antenna for communicating with the tag at a designated RF frequency among several allocated for this purpose. An additional interface such as RS 232, RS 485, or other, may be provided with the reader to allow data received from the tag to be forwarded to another system.
Historically, the RFID tag has constituted a read-only (RO) device, capable of transmitting only fixed, invariable information stored in the tag memory of the integrated circuit chip in which the tag is fabricated, as backscatter readout when the tag is scanned by the reader in an RF communication between reader and tag. More recently, read/write (R/W) RFID tags have been developed, which are adapted to be programmed or altered in memory content by write data received by the tag in the reader's RF beam. Data is stored in the tag memory such as an electrically-erasable programmable read-only memory, or EEPROM, and may consist of original data, an overwrite of previously stored data, refreshed data, and/or entirely new data, which is then available for readout from the R/W RFID tag upon the tag's receipt of a scan (in this example, a command) from a RFID reader.
A RFID reader transmits a continuous wave (CW, i.e., non-modulating) scan at the designated RF communication frequency to interrogate a remote RFID tag, and in response, receives an automatic backscatter RF signal transmitted by the tag, containing RO data stored in the tag's memory. Communication with a R/W RFID tag requires a RFID reader that transmits a modulated RF signal (in contrast to the CW RF signal generated by a reader for acquiring RO data from a tag) by which data is provided to the tag for programming (overwriting) or to initiate other functions of the tag.
A RFID tag that is implemented according to the present invention to perform in either mode of operation, RO or R/W, according to whether it receives a CW or modulated incoming RF command signal from the reader, is termed herein as a dual mode tag. The dual mode tag may be equipped with an EEPROM device to serve both the read-only operation and the read/write operation. As used in this specification, the term “read-only” refers not to the type of memory, but rather, in the RFID sense of a tag that operates, when it is powered up, to send the same data continuously and repeatedly. In the case of a tag that operates only in the RO mode, it does nothing else but to perform this repeated transmission. In contrast, the term “dual mode” refers to a tag's ability to operate in an agile way to sense whether the reader is interrogating or commanding with a CW signal or a modulated signal, and to respond accordingly with read-only operation or with read/write operation, whichever is appropriate.
A dual mode RFID tag is desirable in situations where the same tag is or may be subjected to two or more different applications in the normal course of use or exposure of the item, object or product that bears the tag. For example, a vehicle may be provided with a RFID-tag to allow the vehicle to be machine-recognized as being authorized to participate in a system for automatic collection (as by debiting) of a toll upon the vehicle passing a scanning point at which a remote reader is located, such as at a toll collection station along a highway or an entry point in a parking garage or lot. Or the RFID tag may identify the vehicle to the remote reader as being authorized to enter the grounds of a secure facility or area outfitted with access control gates, so as to automatically raise the gate as the vehicle approaches the gated entry point and is so identified. Other system applications abound, including those in which RFID tagged badges are issued to be worn by employees authorized by various levels of access within a secure facility. And a particular system may be used in several different locations or in several different applications. With the passage of time, the particular system may be upgraded, or planned by its owner or operator to migrate, from a simple read-only technology to a somewhat more complex read/write technology.
In such circumstances, a period of time exists during which RFID readers must be provided to read both legacy (originally issued) RO tags and newly issued R/W tags. A difficulty arises because the newly issued R/W tags will not operate with some or even many of the multiplicity of roadway, parking, airport and garage applications with which the older tags had been designed to operate, unless all of the applications are upgraded concurrently. And the older tags may be inoperable in the upgraded systems or applications. In these instances, it becomes essential to provide tags that are configured to operate in both modes, RO and R/W, to enable usage and proper system performance in all of these applications.
In another application where a dual mode RFID tag is advantageous, a multi-level or multi-step system may be used for enabling or denying access to different areas of a secure facility. The facility may have one level of security for individuals to gain access to an outer portion of the facility, and a higher level of security for access to an inner portion of the facility. At entrances to the lower level of secure access, RFID readers may be used to perform a simple CW scan of employee badges containing a RFID tag, which produces a RO response to allow admission past the security stations. The badges of employees who have been granted the higher security level, however, may utilize dual mode tags that provide not only the RO response at the lower level entrances, but an additional appropriate response to R/W readers stationed at one or more entrances to the higher security portion of the facility.
Thus, at least two distinct advantages exist for providing a dual mode RFID tag, in which the operating modes are both a CW (RO) response and a modulated signal (R/W) response. One of these advantages is the capacity for interoperability between independent applications, and the other is the provision of a distinct migration path from one response type to the other for independent applications.
Prior mode detection schemes for dual mode RFID tags are inefficient and slow to respond, especially to support moderate to high speed applications of the tags and readers, and therefore have been found to lower system margin. System margin is tantamount to a figure of merit based upon the number of times a given type of transaction can be completed or the number of times a given set of frames can be read in a given time interval. A typical frame of interest has a length of 128 bits of data, and includes appropriate frame markers. The greater the number of transactions that can be accommodated in the given time interval, the higher the system margin. High system margin is especially important in moderate to high speed dynamic applications such as “on the fly” toll tracking and collection systems that rely on RFID tag reading as vehicles pass at or near highway speed through a designated unmanned toll collection lane or lanes of a toll plaza.
It would be desirable to detect and comply with a command in an RF signal from a remote reader to a dual mode RFID tag by a method and means of automatic mode detection, and where appropriate, consequent mode switching or mode assumption that optimizes the number of transactions that can be handled in a given time period.
As used in this patent specification, the term “mode detection” is intended to refer to recognition by a dual mode RFID tag of a directive, command or instruction contained in an incident RF signal for the tag to respond in an operating mode consistent with such command; and the term “automatic mode detection” is intended to refer to a tag performing such mode detection, and when applicable, to undergo mode switching from one to the other when it is not already in the proper mode called for by the reader's command, or to undergo mode assumption, promptly and without need for any type of manual intervention. The term “mode assumption” is intended to mean a dual mode RFID tag commencing the correct operating mode from a condition in which the tag is idle, inactive, or shut off, but in readiness to receive the command as an incoming RF signal from a nearby RFID reader. And the terms “mode switching,” and “mode change,” are used interchangeably herein and intended to refer to a dual mode RFID tag undergoing a change in operating mode from RO to RW, or vice versa, regardless of how the switch or change is manifested.